The present invention relates to microwave landing systems (MLS) with separate elevation and bearing (or azimuth) stations.
As is known, the international civil aviation organization (ICAO) has adopted the microwave landing system as a successor to the instrument landing system (ILS) as the landing aid to be used throughout the world in the next decade.
Hereinafter, reference is made to the principle of the MLS. It supplies the aircraft with its position in spherical coordinates in a reference mark linked with the runway, i.e. the elevation angle and bearing angle. The distance of the aircraft is supplied by auxiliary distance measurement equipment (DME) which operates at a different frequency.
The elevation and bearing angles are transmitted on the same frequency of approximately 5 GHz by ground stations close to the runway and a suitable receiver makes it possible to measure these angles on board the aircraft using the system. The elevation and bearing angles are transmitted on a time sharing basis on the same carrier frequency and the measurement is of the anametric type, i.e. it is performed on board various aircraft using the system and this takes place in an independent manner on the basis of a ground transmission.
Each bearing or elevation station, whereof a block diagram is given in FIG. 1, comprises a transmitter 1 with an antenna switch 16 controlled by an electronic control device 2 and an internal monitor 3 checking the quality of the transmission, an electronic scan antenna 4 and a sector antenna 5, an external monitor 6 also being provided for checking the quality of the transmitted signal. The elevation or bearing angular information is produced by a narrow fan-shaped beam 11 along the angular coordinate in question scanning on an outward and return basis the complete angular sector of the coverage at a constant angular velocity V and the angular position of an aircraft is determined on board by measuring the time interval between the reception of the passages of the outward beam and the return beam. This time interval is a linear function of the angular position of the aircraft and the angular information .theta. is equal to V/2 (To-T), in which V is the constant angular velocity of the scan of the angular sector considered by the beam of the antenna, T the time interval between the reception of the outward and return passages of the beam and To the value of the time interval T for a zero angle .theta.. To and V are constants defined by international standards on MLS. Each elevation and bearing angular coordinate is transmitted with the aid of a specific antenna in this system. Thus, there are at least two preferably electronic scan antennas in an MLS. The transmissions on the elevation and bearing antennas are multiplexed in time, together with the identification information and various data on the system, which are transmitted throughout the volume of the angular coverage by sector antennas.
FIG. 2 shows the principle of angular measurement, as described hereinbefore, in the case of a bearing angle.
FIG. 2a shows the scan of the outward beam designated 11 and having as its origin a point A close to the end of the runway, which is the location of the antenna which produces it. Minor lobes are shown in addition to the major lobes forming the narrow measuring beam. It is assumed that beam 11 passes at a given time on aircraft AF. On referring to FIG. 2c which represents the signal S obtained as a function of time, it can be seen that signal S1 given by the outward beam appears at a time t1 and that it exceeds a predetermined measuring threshold SM. In the right-hand part of FIG. 2 it is possible to see the return beam 110, which obviously has the same origin A as the outward beam 11. Signal S2 appears in FIG. 2c at time t2, when return beam 110 passes on aircraft AF. It is pointed out that, by convention, time t1 and/or t2 corresponds to the middle of the signal received S1 or S2. Thus, this time can easily correspond to the middle of the width of the considered signal at - 3 or -6 dB compared with its maximum value. Time 0 at the time origin corresponds to the start of the outward scan of the beam, while time t3 corresponds to the end of the return scan. The time interval separating t2 from t1 is equal to T. A scan stop dead time TM is observed between the end of the outward scan and the start of the return scan.
FIG. 3 shows the principle of the multiplexing of functions in time for a complete MLS station. Part (a) shows how the stations are arranged with the elevation station 7 located close to the starting threshold of the runway and the front bearing station 8 close to the end of the runway. FIG. 3a also shows a rear bearing station 9 close to the starting threshold 100 of runway 10, but this station is not currently used.
Part (b) of FIG. 3 shows the information transmitted by the aforementioned stations on a time sharing basis on the same frequency. The bearing station 8 supplies the front bearing information AZ, as well as the basic data DB and auxiliary data DA, while the elevation station supplies elevation information SI. Part (c) of FIG. 3 shows in greater detail and as a function of time, the information specified in part (b). The front bearing information AZ consists of a preamble PRZ, the outward scan being BA and the return scan BR. The main function of the preamble is to give the elevation or bearing identity of the transmission which immediately follows. Thus, the elevation information SI also has a preamble PRS, an outward BAS and a return scan BRS. The basic data DB also consists of a preamble PRB and the actual data DBA. The same situation as for the basic data applies to the auxiliary data, but they are not shown in FIG. 3c. Obviously, both FIGS. 3b and 3c are arranged relative to time t, whose axis is represented by ot.
It is pointed out that the basic data are considered to be data which are operationally indispensible for landing an aircraft on the runway. The auxiliary data are supplementary data for special operations. For example, the basic data supply the aircraft with information on the geometry and characteristics of the landing system. These data are transmitted by sector antenna 5 and the corresponding signal consists of a preamble PRB and the actual data transmitted in differential phase modulation DPSK.
It is pointed out that the main function of the preamble is to give the bearing or elevation identity of the directly following transmission. A preamble is transmitted by a sector antenna 5 serving the complete coverage of the system. This preamble is generally coded in differential phase modulation (DPSK for differential phase shift keying). The angular information is transmitted by an electronic scan or scanning beam antenna 4.
In practice, for operational reasons, the elevation and bearing stations are several kilometers from one another and, as stated hereinbefore, the bearing station is close to the end of the runway and the elevation station close to the start of the runway. It is pointed out that these notions are relative to the landing direction of aircraft and if the runway is equipped in both directions, there are two totally independent elevation and bearing microwave landing systems. Operation on a single frequency with time sharing consequently imposes a synchronization connection in time between the two stations. Thus, it is necessary to have for each elevation or bearing station a sector antenna and an electronic scan antenna, the sector antenna of a station transmitting the preamble of the corresponding function.
Thus, as a sector antenna is required for each elevation and bearing station, there is a duplication of certain costly elements for a complete system.